The conversion of oxygenates to olefins (OTO) is currently the subject of intense research because it has the potential for replacing the long-standing steam cracking technology that is today the industry-standard for producing world scale quantities of ethylene and propylene. The very large volumes involved suggest that substantial economic incentives exist for alternate technologies that can deliver high throughputs of light olefins in a cost efficient manner. Whereas steam cracking relies on non-selective thermal reactions of naphtha range hydrocarbons at very high temperatures, OTO exploits catalytic and micro-architectural properties of acidic molecular sieves under milder temperature conditions to produce high yields of ethylene and propylene from methanol.
Current understanding of the OTO reactions suggests a complex sequence in which three major steps can be identified: (1) an induction period leading to the formation of an active carbon pool (alkyl-aromatics), (2) alkylation-dealkylation reactions of these active intermediates leading to products, and (3) a gradual build-up of condensed ring aromatics. OTO is, therefore, an inherently transient chemical transformation in which the catalyst is in a continuous state of change. The ability of the catalyst to maintain high olefin yields for prolonged periods of time relies on a delicate balance between the relative rates at which the above processes take place. The formation of coke-like molecules is of singular importance because their accumulation interferes with the desired reaction sequence in a number of ways. In particular, coke renders the carbon pool inactive, lowers the rates of diffusion of reactants and products, increases the potential for undesired secondary reactions and limits catalyst life.
Over the last two decades, many catalytic materials have been identified as being useful for carrying out the OTO reactions. Crystalline molecular sieves are the preferred catalysts today because they simultaneously address the acidity and morphological requirements for the reactions. Particularly preferred materials are eight-membered ring aluminosilicates, such as those having the chabazite (CHA) framework type, as well as silicoaluminophosphates of the CHA framework type, such as SAPO-34.
Chabazite is a naturally occurring zeolite with the approximate formula Ca6Al12Si24O72. Three synthetic forms of chabazite are described in “Zeolite Molecular Sieves”, by D. W. Breck, published in 1973 by John Wiley & Sons, the complete disclosure of which is incorporated herein by specific reference. The three synthetic forms reported by Breck are Zeolite “K-G”, described in J. Chem. Soc., p. 2822 (1956), Barrer et al; Zeolite D, described in British Patent No. 868,846 (1961); and Zeolite R, described in U.S. Pat. No. 3,030,181 (1962). Zeolite K-G zeolite has a silica:alumina mole ratio of 2.3:1 to 4.15:1, whereas zeolites D and R have silica:alumina mole ratios of 4.5:1 to 4.9:1 and 3.45:1 to 3.65:1, respectively.
U.S. Pat. No. 4,544,538, incorporated herein by reference, describes the synthesis of another synthetic form of chabazite, SSZ-13, using N-alkyl-3-quinuclidinol, N,N,N-tri-alkyl-1-adamantylammonium cations and/or N,N,N-trialkyl-exoaminonorbornane as a directing agent in a conventional OH− medium. According to the '538 patent, SSZ-13 typically has a silica to alumina molar ratio of 8 to 50, but it is stated that higher molar ratios can be obtained by varying the relative ratios of the reactants in the synthesis mixture and/or by treating the zeolite with chelating agents or acids to remove aluminum from the zeolite lattice. However, attempts to synthesize SSZ-13 in OH− media at silica to alumina molar ratios in excess of 100 have been unsuccessful and have produced ITQ-1 or SSZ-23, depending on the alkali metal cation present. Moreover, increasing the silica to alumina molar ratio of SSZ-13 by dealumination has met little or no success.
Significant work has been conducted on the use of SSZ-13 as a catalyst for MTO reactions. However, investigations to date have tended to show that the performance of SSZ-13 is inferior to that of its silicoaluminophosphate analog, SAPO-34. See, for example, Yuen, L.-T., Zones, S. I., Harris, T. V., Gallegos, E. J., and Auroux, A., “Product Selectivity in Methanol to Hydrocarbon Conversion for Isostructural Compositions of AFI and CHA Molecular Sieves”, Microporous Materials 2, pp. 105-117 (1994) and Dahl, I. M., Mostad, H., Akporiaye, D., and Wendelbo, R., “Structural and Chemical Influences on the MTO Reaction: A Comparison of Chabazite and SAPO-34 as MTO Catalysts”, Microporous and Mesoporous Materials 29, pp. 185-190 (1999).
In U.S. Published Patent Application No. 2003/0069449, published Apr. 10, 2003, Zones, et al., describe improved MTO performance with SSZ-62, which is essentially the small crystal version of SSZ-13. The same N,N,N-tri-methyl-1-adamantylammonium organic template was used for the synthesis of SSZ-62 as for SSZ-13.
U.S. Pat. No. 6,162,415 discloses the synthesis of a silicoaluminophosphate molecular sieve, SAPO-44, which has a CHA framework type in the presence of a directing agent comprising cyclohexylamine or a cyclohexylammonium salt, such as cyclohexylammonium chloride or cyclohexylammonium bromide.
U.S. Published Patent Application No. 2004/0253163, published Dec. 16, 2004, discloses the synthesis of silicoaluminophosphate molecular sieves having the CHA framework type employing a directing agent having the formula:R1R2N—R3 wherein R1 and R2 are independently selected from the group consisting of alkyl groups having from 1 to 3 carbon atoms and hydroxyalkyl groups having from 1 to 3 carbon atoms and R3 is selected from the group consisting of 4- to 8-membered cycloalkyl groups, optionally substituted by 1 to 3 alkyl groups having from 1 to 3 carbon atoms; and 4- to 8-membered heterocyclic groups having from 1 to 3 heteroatoms, said heterocyclic groups being, optionally, substituted by 1 to 3 alkyl groups having from 1 to 3 carbon atoms and the heteroatoms in said heterocyclic groups being selected from the group consisting of O, N, and S. Preferably, the directing agent is selected from N,N-dimethyl-cyclohexylamine, N,N-dimethyl-methyl-cyclohexylamine, N,N-dimethyl-cyclopentylamine, N,N-dimethyl-methyl-cyclopentylamine, N,N-dimethyl-cycloheptylamine, N,N-dimethyl-methylcyclo-heptylamine, and most preferably is N,N-dimethyl-cyclohexylamine. The synthesis can be effected with or without the presence of fluoride ions.
In a paper entitled “Templates in the transformation of zeolites to organozeolites. Cubic P conversions”, ACS Symposium Series (1990), 437 (Novel Mater. Heterog. Catal.), 14 to 24, Zones et al. disclose that the treatment of low-silica zeolite P with a RMe3NI directing agent, where R is cyclohexyl, 2- or 3-methylcyclohexyl; 2-exo- or 2-endo-norbornyl, 2-bicyclo[3.2.1]octyl, 9-bicyclo[3.3.1]nonyl, or 1- or 2-adamantyl, leads to the crystallization of high-silica SSZ-13 containing the directing agent.
Regular crystalline molecular sieves, such as CHA framework-type materials, are built from structurally invariant building units, called Periodic Building Units, and are periodically ordered in three dimensions. However, disordered structures showing periodic ordering in less than three dimensions are also known. One such disordered structure is a disordered planar intergrowth in which the repeated building units from more than one framework type, e.g., both CHA and AEI, are present. In addition, for certain molecular sieves, the building units can exist in mirror image forms, which can result in stacking faults where a sequence of building units of one mirror image form intersects a sequence of building units of the opposite mirror image form.
U.S. Pat. No. 6,334,994, incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, referred to as RUW-19, which is said to be an AEI/CHA mixed phase composition. In particular, RUW-19 is reported as having peaks characteristic of both CHA and AEI framework-type molecular sieves, except that the broad feature centered at about 16.9 (2θ) in RUW-19 replaces the pair of reflections centered at about 17.0 (2θ) in AEI materials and RUW-19 does not have the reflections associated with CHA materials centered at 2θ values of 17.8 and 24.8.
U.S. Pat. No. 6,812,372, incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve comprising at least one intergrown phase of molecular sieves having AEI and CHA framework types, wherein said intergrown phase has an AEI/CHA ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of said silicoaluminophosphate molecular sieve.
CHA framework type, and CHA/AEI intergrown, molecular sieves appear to be particularly suitable catalysts for the OTO reaction since they have cages that are sufficiently large to accommodate aromatic intermediates while still allowing the diffusional transport of reactants and products into and out of the crystals through regularly interconnected window apertures. By complementing such morphological characteristics with appropriate levels of acid strength and acid density, working catalysts are produced. Extensive research in this area indicates that silicoaluminophosphates are generally more effective OTO catalysts than aluminosilicates. In particular, increasing the silica to alumina molar ratio seems to be a key requirement in the use of aluminosilicates in OTO reactions.
For example, an all silica crystalline molecular sieve having the CHA framework type has been hydrothermally synthesized using N,N,N-trimethyladamantylammonium in hydroxide form as the structure-directing agent at nearly neutral pH in the presence of fluoride. See Diaz-Cabanas, M-J, Barrett, P. A., and Camblor, M. A. “Synthesis and Structure of Pure SiO2 Chabazite: the SiO2 Polymorph with the Lowest Framework Density”, Chem. Commun., 1881 (1998).
More recently, an aluminosilicate with the CHA framework type and having a silica to alumina molar ratio in excess of 100, such as from 150 to 2000, has been synthesized in the presence of fluoride ions. The directing agent employed is selected from N-alkyl-3-quinuclidinol cations, N,N,N-tri-alkyl-1-adamantammonium cations and N,N,N-trialkyl-exoaminonorbornane cations. See U.S. Patent Application Publication No. 2003/0176,751, published Sep. 18, 2003, and incorporated herein by reference.
U.S. Pat. No. 7,094,389, incorporated herein by reference, discloses a crystalline material comprising a CHA framework-type molecular sieve with stacking faults or at least one intergrown phase of a CHA framework-type molecular sieve and an AEI framework-type molecular sieve, wherein the material is substantially free of framework phosphorus and has a composition involving the molar relationship (n)X2O3:YO2 wherein X is a trivalent element, Y is a tetravalent element and n is from 0 to about 0.5. The material can be synthesized using a mixed directing agent comprising an N,N,N-trialkyl-1-adamantylammonium compound and an N,N-diethyl-2,6-dimethylpiperidinium compound, normally in the presence of fluoride ions.
There is, however, interest in finding improved methods of synthesizing silicates and high silica aluminosilicates having the CHA framework type or a CHA/AEI intergrown framework and, in particular, methods which avoid or minimize the use of HF and expensive structure directing agents.